Abrasives



D. R. ADA

ABRASI Filed Nov.

Feb. 2, 1965 VES United States Patent O 3,l63,387 ABRASIVES Donald R. Adams, Santa Monica, Calif. (9000 Sunset Blvd., Suite 319, Los Angeles, Calif.) Filed Nov. i7, 1959, Ser. No. 853,582 3l Claims. (Cl. 51-295) This invention relates to abrasives, and more particularly to improvements in abrasive articles of the type made from a plurality of abrasive granules, such as coated abrasive papers, grinding wheels and the like.

Abrasive articles made by bonding a large number of relatively small granules of solid abrasive materials, either to a backing such as a paper, fiber, cloth or metal sheet, or formed into a coherent mass such as a grinding wheel or block or a cutting wheel, are extremely common in industry and even household use.

In spite of much research and development effort which has gone into the articles of this type, these abrasives are not always perfect, and suffer from a number of disadvantages. For example, in the case of grinding wheels more abrasive particles are consumed than actually go into the grinding or cutting operation itself; otherwise stated, the grinding efliciency is far from 100% considering the potential abrasive material present. This comes about as a result of chipping of individual abrasive granules, as well as the dislodgernent of abrasive granules from the matrix in which they are contained before they have been consumed by normal grinding. The same difficulties appear in coated abrasives such as emery cloth, sand paper, and the like, and indeed generally to an even higher degree. Commonly, the abrasive granule layer on coated abrasives is only one particle thick, and long before individual particles are worn down to even half of their original size, they are often dislodged from their bond to the backing, or the backing itself becomes partially destroyed or damaged in portions of the sheet material so that it must be discarded. Another disadvantage common to the abrasive materials described is that they are all subject to maximum limits of grinding speed. As attempts are made to grind faster than a maximum rate which depends upon the particular abrasive article used and the material being ground or abraded, then over-heating rapidly leads to an excessive rate of consumption or even destruction of the abrasive article. Maximum grinding rates can be increased by wet grinding as distinguished from dry grinding, wherein a coolant fluid is used, such as water, but even in these cases localized heating at the actual cutting surface of the abrasive granule sets an upper limit to grinding speeds.

It is an object of this invention to increase the durability of abrasive articles.

Another object of the invention is to provide a method for increasing the maximum grinding speed for a given abrasive article.

Another object of the invention is to provide abrasive articles which develop less frictional heat when used in a grinding process.

A further object of the invention is to provide abrasive articles having enhanced heat dissipating properties.

Another object of the invention is to provide an abrasive article which exhibits decreased consumption of abrasive material when in use, while yet maintaining eicient grinding action.

Other objects of the invention will appear as the description thereof proceeds.

Generally speaking, and in accordance with an illustrative embodiment of the invention, the objects thereof are accomplished by providing a relatively thin adherent coating containing a metal-leaf pigment, as hereafter defined, on the exposed surfaces of abrasive granules contained either as a coating on a sheet or other substratum, or

ICC

contained in a coherent mass as in a grinding wheel, the coating likewise being applied to any bonding material serving to hold the individual abrasive granules in place.

A description of the invention will be facilitated by reference to the drawings, in which:

FIGURE 1 is a cross-section of an abrasive article of the so-called coated type.

FIGURE 2 is a cross-section of a coated abrasive article as in FIGURE l, after use.

FIGURE 3 is a cross-section of a portion of an abrasive article of the coherent type such as a grinding wheel, after use.

FIGURE 4 is a sectional view of a coatedabrasive with a backing layer to be described.

The abrasive particles, such as indicated by reference characters It) and 30 in FIGURES 1, 2 and 3, are coin'- monly flint, garnet, alumina, silicon carbide, crushed glass, crushed quartz, or the like. For sandpapers, the relatively cheap ilint and garnet are widely used, while in grinding wheels, the better types of coated abrasive cloth, and the like, alumina, tungsten carbide, and silicon carbide may be used. Crushed diamonds may also be used, as well as boron carbide. My invention is applicable to all such abrasives, including but not limited to those specically mentioned. The form of the abrasive particle is almost always substantially isodimensional, that is, the abrasive particles are neither ilakes nor fibers, but the individual particles have dimensions in all three coordinates of about the same order of magnitude for a given particle, even though some of the particles may be somewhat elongated. The actual size of the particles varies considerably depending upon the abrasive article desired and the type of service contemplated. In general, abrasive sizes in common use range from about l0 mesh to about 200 mesh and finer. Again, my invention is applicable regardless of size, as may be seen from the latter description herein. n In order for the abrasive particles to abrade theobjects to be ground or otherwise worked on, it is necessary to hold the abrasive particles in place. This is accomplished by means of a bonding agent. Those in common use are quite diverse, ranging from hide and starch glues for cheaper types of sandpaper, through other organic adhesives such as varnish-type vehicles, shellac, alkyd resins, various phenolic resins, polyester adhesives, natural and synthetic rubber adhesives and various other synthetic plastics, to inorganic bonding agents such as sodium silicate, other silicates, ceramic bodies and the like. In some cases the abrasive particles may be made to cohere by raising the temperature of a compressed mass of particles until sintering takes place. In this case, the abrasive particle itself becomes its own bond, and in this'description and in the claims which follow, the term bonding materia or bonding agent will be understood to include the abrasive material itself when caused to act in this manner. The very hardest abrasives, however, tend to have very high melting points, so that it is more common to use a bonding agent other than the abrasive particle material itself. Where a sheet or plate of metal is used as a support for the abrasive granules, the latter can of course be attached by kany of the adhesives already mentioned, and may even'be brazed to the metal with another metal o-r metallic alloy of lower melting point. Thus, silicon carbide granules may be brazed with copper to a steel sheet.

The abrasive particles may be bonded to `a suitable backing, or they may be bonded to themselves. In the specific embodiment shown in FIGURE l, the abrasive particles 10 may be for example silicon carbide, while the backing sheet 11 may be any suitable paper, cloth, ber, metal or like sheet, most generally solid, but sometimes in mesh form. It may be noted that paper and 3 cloth are the commonest backing materials, and are generally heavily treated with various sizing coats so as to possess llexibility, toughness, and in some cases resistance to water, and also so as to present a suitable surface to which the abrasive particles may be bonded. It is not necessary to describe the various treatments commonly applied to sheet backing materials in View of the fact that these are Well-known to the art and that my invention is applicable to all coated abrasives regardless f the nature of the backing material. i

Where .the abrasive particles are coated on to a sheet material, the bonding agent generally forms a layer on the coated side immediately adjacent to and adherent tol the backing material. Such a layer of bonding material'is indicated by reference character 12 in FIGURE 1. The abrasive particles are firmly embedded in yandheld by the bonding material 1,2. This is generally accomplished by applying the abrasive particles to the bonding material while it is still in a iiuidV or semi-fluid, sticky condition, and moreover by applying one or more light v coats of additional bonding or sizing material, generally in a suitable solvent, or in some cases merely a coating of bonding materialsolvent itself, to the coated abrasive so, that the bonding agent tends to grip at least a portion of the sides of the abrasive particles as well as those portions in substantially direct contact with the backing material. This condition is clearly evident in FIGURE 1 Where a thin layer of bonding material is shown on most of the particles, for example, at the point indicated by 13, but may be absent from the generally exposed portions of some of the grains, as in the particle indicated by reference character 14 in FIGURE 1. It isimmaterial for the practice of Vmy invention whether the abrasive particles `are exposed completely over some of :their surface or are fairly well covered by a thin coatingr of bonding material. 1

Whererthe abrasive article is a coherent mass of particles, as for example in the case ofgan abrasive Wheel, an abrasive'block or whetstone, or the like, the bonding material generally joins the portions of the particles which are in actual or approximate contact, with void space generally left in between the particles. Thesituation in most abrasive articles of this type is quite analogous to' a mass of sand with a relatively small amount of water, wherein the water is present at the points of contact of the sand ,particles but'not elsewhere. In fact, the circumstance Vthat the bonding agent is` generally inra liquid or. semiliquid formY at some stage of the manufacture causes it to assume the same position that Water does in a mass of sand and indeed for the same reason, iLe. capillarity. In FIGURE 3, the void spaces 33 appear between the particles, which in turn are joined by bonding material 32.

It should be noted that FIGURES l, 2, 3 and 4 are not only sections, they are indeed thin sections or slices through the articles in question, this having been done for clarity of presentation.

As mentioned, Yin accordance with my invention, the exposed portions of the abrasive particles, together with any exposed portions of bonding agent, in abrasives of the types described, are coatedv with a thin layer of a metal pigment, and more particularly a metal-leaf pigment,in a suitable vehicle.V

A wide variety of metals may be used, although I prefer those that have corrosion Vresistance and in general some ductility. Hardness is not especially necessary or desired, and indeed relatively soft metals are very useful, although readily fusible metals are not desirable. Y Suitable metals include aluminum; aluminum alloys, especially with beryllium; copper; a'wide varietyl of copper alloys including copper-zinc, copper lead and copper-zinc-lead alloys, thus including the various brasses and bronzes;

beryllium copper; stainless steel; nickel; nickel alloys;V

gold, silver; they platinum metals; and the like.

The metal-leaf coatings are shown by reference characters and 16 in FIGURES 1 and 2, 31 in FIGURE 3, and 41 in FIGURE 4.. v

A method which is quite generally applicable to abrasivesof the type described utilizes Ythe desired metal or indeedV mixture of metals as a metal-leaf pigment in a vei hicle, such as a varnish or lacquer base. vThe metal-leaf pigment is lapplied in such a way that it is adherent to the abrasive particles, which may readily be accomplished by including a small amount of asuitable binding agent with the metal-leaf pigment. In general, formulations for aluminum varnish or bronze-leaf varnish and simi-lar coatings generally may be used. For simplicity in presentation, it will be understood that when an adherent layer of a metal-leaf pigment is mentioned herein, particularly in the claims, itis -to be understood that sucient of a suitable binding agent, such as a varnishV or lacquer vehicle,

Vis present in the formulation to cause adherence yof the metal-leaf pigment to the surfaces so treated, and coherence of .the pigment particles to each other. The metal-leaf pigment may be combined with the so-called size coats, that is, the adhesive coatings applied to the abrasive granules after they have been embedded in the maker coat, or the metal-leaf pigment may be applied -as a separate coating after the size coat or coats have been applied, or it may even replaceV the size coat or coats themselves.

All of the metals which are commonly and generally employed in surface coatings of this type, wherein the metal is formed into a metal-leaf pigment and dispersed in a suitable coating or suspending vehicle, may be used. Aluminum and the various brasses, known in the metalleaf pigment art as bronzesf stainless-steel leaf pigments, and nickel leaf pigments are available and suitable. Such metal-leaf pigments are most commonly made by comminuting the desired metal, as by spray atomizing'in the molten condition, followed by ball-milling so that the individual metal particles are flattened out into a flake or leafrforrn; or they may be'produced by a stamping process. Some fatty organic material such as stearic acid or tallow is commonly used not only to assist in the ball-milling process but also to enhance the leang action when mixed with the Vehicle.

Of .the metal-leaf pigments available, the aluminum and copper Vleaf pigments are generally pure aluminum and pure copper respectively. Stainless steel leaf pigment is generally 18-8 chromium-nickel alloy.V The copper-'zinc alloys contain from 10 to 35% zinc, while the copperzinc-aluminum alloys contain up to 15% each of zinc and aluminum in addition to the copper In the practice of my invention I fdo not confine myself to the useof a single metal or alloy in a particular application; I have found that various mixtures of these are specially suitable in the practice of my invention. For example, the following mixtures Acan ,be used, the figures denoting parts by weight in the different lettered mixtures:

Metal ofthe Leaf Pigment B C D E F G H I Aluminum 1 1 1 1 1 Copper 2 1 1 1 1 Copper-zinc 1 Copper-zine-aluminum- Stainless steel 2 1 Nickel Y I choose metal-leaf pigment particles considerably smaller in all dimensions than the granules of abrasive to be coated. This is readily done since the average metal-leaf pigment particle is substantially smaller than the average abrasiveparticle size, and for abrasive granules of small particle size it is merely necessary to use a particularly ne metal-leaf pigment. The reason that this is done is so that the metal-leaf pigment coating may readily conform to the surface of the abrasive granule or particle and form a coating which is thin relative to the particle size. A thickness of the adherent layer, containing the metal-leaf pigment particles, 1/2 to 1,4300 the average abrasive particle diameter will be found particularly suitable, although I do not mean to limit myself in this respect.

It will of course be understood that when a layer of metal-leaf pigment particles is laid down in the fashion described, because of the extreme thinness of the individual flaky metal particles there will be a number. of particles comprising the thickness of the layer. Accordingly, when a metal-leaf pigment layer is recited in this specification and in the claims which follow, it will be understood that this refers to a layer made up of a multiplicity of overlapping metal iiakes, in which the layer itself is a number of times the thickness of the individual iiakes.

Where an abrasive article of the coated type is to be processed in accordance with the invention, using a metalleaf pigment, the varnish or lacquer met-leaf pigment formulation can be applied by usual and known means such as application by brush, spraying, dipping, roll-coating, and the like. Where metal-leaf pigment is to be applied to a coherent mass of granules as is the case with a grinding wheel, then the grinding wheel may be dipped in the suspension containing the metal-leaf pigment. Where the average pore space in the abrasive article is large, simple dipping snfces for considerable penetration and coating of the walls of the pores within the mass of abrasive particles. However, this action is facilitated by utilizing pressure to force the metal particles into the abrasive article. For example, where a number of grinding wheels are to be treated, they are placed in a pressure container and covered with metal-leaf pigment formulation, whereupon pressure is applied to the container, thus forcing the liquid containing the metal-leaf pigment into the pores. Upon release of pressure, much of the liquid will be forced out again by air compressed within the pores, but this will still leave an adherent metallic coating on the particles. The excess liquid may be removed from the grinding wheels by centrifugal force. Drying or setting of the remainder takes place as usual.

Extensive tests have proved the peculiar and unexpected advantages arising from the metal-leaf pigment coating as described in accordance with the present invention. Examination of both coated abrasive and coherent granular abrasive articles such as grinding Wheels and abrasive blocks which have been treated in accordance with the invention show some wholly unexpected and valuable phenomena. In the first place, the individual abrasive granules tend to wear down more gradually and cleanly and only on their outermost projections, that is, on the portions of the granules actually exposed to the workpiece. The metal-leaf pigment layer prepared in accord- `ance with the invention remains intact, even under conditions of severe use. Of course, as an individual abrasive granule gradually wears away, the metal-leaf pigment coating wears along with it, but not to any appreciable extent below the height of the actual working abrasive surface. This situation is shown in FIGURES 2 and 3, where the truncated tops of the abrasive particles shown `are the result of use of the abrasive article in actual grinding work.

The mechanism of the abrading away of abrasive granules in actual use is noty fully known, but it is believed by some workers to consist of a series of microscopic chipping actions. Without desiring to be held to any `theory of action, I believe that the longer life of abrasives made in accordance with the present invention may at least in part be ascribed to the control over this chipping action which is brought about by the presence of the metal-leaf pigment layer. This layer will tend to prevent gross chipping away of an abrasive granule, by the pro- While I do not wish to be limited to any theory ofl action, it seems clear, after studying the results obtained in practical tests, that the metal-leaf pigment layer performsV at least two functions which bear directly on the superior results obtained. In the rst place, the metalleaf pigment layer, remaining as it does intact up to the actual cutting edges of the particles, and having an extremely high metal content, serves as an excellent heat conductor and substantially reduces the temperature obtained at the cutting edges of the granules while in use. While the metal-leaf pigment layer is thin, the distances involved are short, and since it is ever so much better a conductor of heat than the materials of the commonly used abrasive granules, bonds and sizing, the enhanced cooling action is appreciable.

Another important role played by the metal-leaf pigment layer is to form a heavy-duty solid lubricant for the sides and even the cutting edges of the particles. This is especially so when the metal of which the pigment is composed is relatively soft and ductile, as is the case for example with aluminum, aluminum alloys, copper, and copper alloys; but lubrication is still appreciable in the case of nickel and stainless steel metal-leaf pigments, which doubtless arise from the small particle size and the flaky character of the particles. This lubricating action is important in preventing undesired seizure of the abrasive particles, whereby they might otherwise be dislodged or prematurely chipped away. Because of the at nature of the metal-leaf pigment particles, considerable orientation of the flakes parallel to the adjacent abrasive particle surface takes place. In actual use the metal-leaf pigment layer tends to become burnished or polished, thus lowering even more the coeicient of friction to the workpiece as well as the grinding debris which ordinarily collects in the interstices between the abrasive granules. A startling eifect produced in abrasive articles made in accordance with the invention is the almost complete elimination of any tendency for the abrasive article to block up or glaze, that is, to retain the grinding debris in thel abrasive article instead of allowing it to be carried away by the various forces operating during grinding. This again is a consequence of the lubricating action of the metal-leaf pigment layer, as well as its smooth surface, and heat conducting properties.

The action of metal-leaf pigments appears to be unique in this regard, for the covering of the abrasive particles by various mineral pigments, such as the various lead, zinc, and titanium oxides and silicates, zinc and barium sulphates and carbonates, Aand calcium carbonates and silicates, as used in paints, for example, does not lead to this desirable effect at all, and indeed worsens matters. While again I do not wish to be bound to any one theory of action, it appears that this stems from the fact that mineral paint pigments of this type lack both the solidlubricant property and the good heat conductivity discussed above. K

In some cases, when dealing with coated abrasives, it will be found advantageous to apply a metal-leaf pigment coating of the type described to the back side of the backing, as well as to the yfront side which contains the abrasive granules. This is shown in section in FIGURE 4, wherein 40 is an adherent metal-leaf pigment layer on the back of a coated abrasive article. This is a convenient means of reducing friction on the backing and -of reinforcing it, and furthermore encloses the entire coated abrasive article within an imperious sheath. When suesser the backside of the backing is so coated as has been'described, another advantage accrues in better conduction of heat away from Vthe actual portions of the abrasive article'which are in grinding service at thertirne. Shelf life of the article is also increased in this manner.

Some Working examples willnow be given.

v Example V1 was immersed for a period'of five minutes` in a mixturer of the following ingredients, and prepared in the following proportions: two -pounds of aluminum metal-leaf pigment conforming to Federal Specification TT-A-A-68a, Type I, Class B, per one gallon of varnish conforming to Federal Specication 'MLV-109, and three pints of mineral spirits conforming to TT-T-29la, Grade l. Upon removal from the suspension the excess material was spun olfby centrifugal force, and the remaining layer was Ithen allowed to dry. Grinding tests on a variety of soft, medium, and hard steels, using an untreated wheel of otherwise like specification as a control, disclosed several advantages derived from the application of the metalleaf pigment layer, viz., a substantially reduced rate of wear, improved grinding action,including better finishes on the steels dealt with, and the substantially complete elimination of any tendency to accumulateor retain grinding debris within the pores of the Wheel.l

Example d ing, one sheet ofteach grit size was cut into three equal size strips of three and `two-thirds inches by nine inches each, to fit a rvibratory sanding machine, and four 5"- discs, two of'each grit size, were cut from two other sheets, vfor use on a portable power sander, using a conventional resilient back-up pad. All were used for sanding, variously,'ro'ck maple, White oak, yelow pine, and fillings-of automobile body solder contained in a repair on a sheet vsteel automobile fender, as well as adjoining portions of the fender itself. ln all cases the abrading action was more effective than with similar strips andfdiscs which had not been coated. The usual tendency for abrasive articles of this type to lill-in between the abrasive'particles Vwith the material being abraded was substantially reduced, producing faster 'and-cleaner abrading action, increased tearV resistance, and extended the serviceability of the treated abrasive strips and discs.

Example 4 v mersed in a fluid mixture conforming to thefollowing sive having a grit number of 100 as defined in paragraph 3.5.2 of said specification; and a cloth backed abrasive disc, ten inchesin diameter and with no center hole,

' coated on one side with aluminum oxide having a grit number ofVSO as above delined', these materials also conformingto the foregoing specication, and designated as Type I, Class I, therein; were spray-coated on their abra- Vsive yside with a fluid mixture consisting of two pounds materials surfaced consisted of Vvarious metals and'woods ranging from medium to hard to very hard, and` the tests included treatment of flat surfaces, curves, langles and edges of the various woods and metals. The treated belt and disc showed a number of improved properties, in-

cluding increased durability, performance, and economy.

' Better nished surfaces were evident, increased ease of operation was noted, Aand less evidence of wear, loading, 'and glazing was observed. i

Example 3 I Aluminum l,oxide V*coated-abrasive paper sheets,V

9" x lli, some having a grit number of 60, and others havinga grit number of 120, the abrasive and the Vgrit sizes conforming to P-C-45l-A, Type I, ofExample 2,v were roll-coated with acopper-zinc bronze metal-leaf pigmented vehicle blend,'prepared in the proportionsrof three pounds f of bronze pigment as described in Example 2, to one gallon of Vmixing varnish conformingto Federal Speciiication TTQV-119, and one quart of'turpentine. After dryproportions ofingredients; one gallon of the varnish vehicle of Example 3, one. pound of the aluminum metalleaf pigment of Example l, one pound of copper-zinc bronze metal-leaf pigment of Example 2, and one pound of 325 mesh stainless steel metal-flake pigment of 18-8 chromium-nickel steel, plus one-half gallon of xylene. The disc was kept submerged in the mixture for a period of twoV minutes, during which time Va pressure of 40 pounds per square inch was applied, thereafter, the disc was removed, the excess fluid mixture thereon was spun oil by centrifugal force, and the remainder permitted to dry. Tests were Vthen made on carbon steel, alloy steel, and on hard cast bronze, with results markedly superior to those of the untreated control disc of otherwise like specilication, in abrasive durability and grinding eihciency.

Example 5 c Three garnet coated abrasive sanding discs, nine inches 1n diameter and having no center hole, cut from three garnet paper sheets-conforming to Federal Specification P-P-l2la, Class l, with-a grit number of 1/0-80 as delined in Vparagraph 3.1.7 of said speciticatiomrwere brushcoated on their abrasiversurfaces-with a metal-leaf pigair dry for a period of 48 hours, after which time they were afxed adhesively, one at atime, to a metal sanding `plate for use on a power disc sander.` Tests were made using pine, r, and cedar. In all three cases, marked superiority was shown over the three control sanding discs, cut from like garnetrsheets but minus the metal-leaf pigment coating. V rl`his Was evidenced from the standpoint of durabilityv andV overall rate of Wear of the inventive articles over relatively long periods yof use. Substantially reducedV was` the normaltendency to"llup with particles of the material being sanded, thus eliminating to al considerable extent the usual and objectionable inclination to glaze, snag, and tear, excessively or prematurely.

'l Example 6 An aluminum oxide coated abrasive belt, 6 x 48,

tion of Example 2, relating to belts, was roll-coated on the underside, that is the cloth side, as well as on the abrasive surface, with the metal-leaf pigment-varnish mixture used and described in Example 1. The rate of wear of the belt during grinding tests made, Working steel, cast bronze, and cast alloy aluminum, was found to be appreciably lower than was a similar belt to which the backing layer had not been applied. In addition, freer, cleaner, and cooler cutting was also observed, especially, the work done at the arc areas of contact, at either extremity, where belt and drum are in immediate and continuous contact, and Where heat is most easily generated in a belt sanding procedure.

Example 7 Flint-coated abrasive paper sheets equivalent to Federal Specification P-P-105, Class 1, and Class 2, with grit designations of Fine (F), Medium (M), and Coarse (C), as defined in Table III, page 3, of said specification, were rst spray-coated with a uid mixture prepared in the following proportions: per three pounds of `the stainless steel metal-flake of Example 4, one gallon of the varnish vehicle and one quart of the solvent-thinner of Example 5. This was followed by a spray application of the aluminum metal-leaf pigment mixture of said Example 5, and the sheets were then allowed to air-dry. When tests were made, using untreated flint abrasive paper sheets of the same specification as controls, on sawed wood, primed wood, painted wood, and liberboard, employing hand work, as well as a portable power sander using discs cut from the abrasive papers, and also, with the power sander on shoe sole leather, substantial increases in abrasive life and cutting quality were observed, as were decreases in loading and glazing.

Example 8 A silicon carbide grinding'wheel, vitried bond, eight inches in diameter, having a face thickness of one inch, and a A; inch center arbor opening, composed of abrasive grain size 60, meeting U.S. Department of Defense MIL. STD. 165, page 15, and designated C60G8V in accordance therewith, was immersed in a fluid consisting of a heat resistant silicone resin varnish vehicle, pigmented with two pounds of aluminum metal-leaf pigment particles per one gallon of vehicle, conforming to U.S. Military Specification MIL-P-14276 (Ord), and diluted with twothirds gallon of toluene. The wheel was allowed to remain submerged for a period of two minutes, during which time a pressure of 50 pounds per square inch was applied. It was then removed from the mixture, the excess uid immediately spun olf by centrifugal force, and the remaining residue was allowed to dry. Thereafter, grinding tests on low tensile strength materials including cast iron, unannealed malleable iron, 18-8 stainless steel, aluminum, brass, marble, ceramics, and glass, were made using an untreated wheel of otherwise like specification as a control. The treated wheel performed in a superior manner as regards freer and cleaner grinding, durability, and over-all efficiency, including liner resultant surfaces.

The best method of practicing the invention of which I am aware is the use of aluminum metal-leaf pigment of the type described, and in a varnish vehicle of the type described, both as in Example 8 hereinabove, applied to a silicon carbide grinding wheel. However, the application of my invention for example to coated abrasives as set forth in Example 2 shares fully in the advantages of the invention, so that Example 2 may be termed nearly the best mode as well.

As has been explained hereinabove, this invention is broadly applicable to particles of an abrasive substance which are held in fixed relationship to one another, such as for example by being bonded to a cloth, paper or metal backing, or by being bonded in the mass as in a grinding wheel. This is broadly distinguished from a slurry of a powdered abrasive material in a liquid such as water, as

is used in certain types of metal grinding, in storie cutting, and the like. As used herein and in the claims which follow, accordingly, the term abrasive article of Vmanufacture includes all such articles of the type described wherein the particles of abrasive granules are fixed with relation to each other by the various means described. The invention obviously is not applicable to a slurry of freely mobile abrasive particles forming part of a liquid slurry, and by the same token such a slurry is not an abrasive article of manufacture either in common parlance, or in the terminology of the art, and of this specification and of the claims appended hereto.

Having described the invention, I claim:

1. An abrasive article of manufacture, comprising, a plurality of granules of an abrasive substance, portions of said granules being exposed and other portions of said granules being attached to a bonding material, said exposed portions of said granules and exposed portions of saidbonding material being coated with an adherent layer of a metal-leaf pigment.

2. The Iarticle of manufacture of claim 1 in which the metal is aluminum.

3. The article of manufacture of claim 1 in which the metal is copper.

4. The article of manufacture of claim 1 in which the metal is a copper-zinc alloy.

5. The article of manufacture of claim 1 in which the metal is a copper-zinc-aluminum alloy.

6. The article of manufacture of claim 1 in which the metal is stainless steel.

7. An abrasive article of manufacture, comprising, a plurality of granules of an abrasive substance, portions of said granules being exposed and other portions of said granules being attached to -a bonding material, said exposed portions of said granules and exposed portions of said bonding material being coated with an adherent layer of a metal-leaf pigment, said layer being a thickness of about 1/2 to about 1/200 of that of the diameter of said granules.

8. A coated abrasive, comprising, a sheet backing, a plurality of granules of an abrasive substance bonded to said backing with a bonding material, said granules and said bonding material being coated with an adherent layer of a metal-leaf pigment.

9. The coated abrasive of claim 8 in which the metal is aluminum.

10. The coated abrasive of claim 8 in which the metal is copper.

11. The coated abrasive of claim 8 in which the metal is a copper-zinc alloy.

12. The coated abrasive of claim 8 .in which the metal is a copper-zincaaluminum alloy.

13. The coated abrasive of claim 8 in which the metal is stainless steel.

14. The coated abrasive of claim 8 in which the layer has a thickness of about 1/2 to about 1/200 of that of the diameter of said granules.

15. A coated abrasive, comprising, a sheet backing having a front side and a back side, a plurality of granules of an abrasive substance bonded to the front side of said backing with a bonding material, said granules and said bonding material and said back side being coated with an adherent layer of a metal-leaf pigment.

16. The coated abrasive of claim 15 in which the metal is aluminum.

17. The coated abrasive of `clai-m 15 in which the metal is copper.

18. The coated abrasive of claim 15 in which the metal is a copper-zinc alloy.

19. The coated abrasive of claim 15 in which the metal is a copper-zinc-aluminum alloy.

20. The coated abrasive of claim 15 in which the metal is stainless steel.

21. An abrasive article of manufacture, comprising, a coherent mass of granules of an abrasive substance, said granules being bonded at their points of contact bon-ding material and presenting open spaces between said granules whereby portions of said granules and portions of said bonding materials are ex-posed, said exposed portions of said granules and said exposed portions of said bonding material being coated withv an adherent layer of a metal-leaf pigment. V

22. The abrasive article oficlaim 21 in which the layer has a thickness of labouti/2 to about 1/200 of that of the dia-meter of said granules.

23. The abrasive article of claim 21 in which the metal is aluminum.

24. The abrasive article of claim 2l in which the metal is copper.

25. The abrasive article of claim 2l in which the metal is a copper-zinc alloy.

26. The abrasive article of claim 2l in which the metal is a copper-Zirrc-aluminum alloy.

27. The abrasive article of claim 2l in which the rnetal isstainless steel.

28. An abrasive article of manufacture, comprising, a plurality of granules of an abrasive substance, portions of said granules being exposed and other portions of said granules being attached to a bonding material, said exposed `portions of said granules and exposed portions of said bonding material being coated with an adherent layer of a metal-leaf pigment chosen lfrom the group consisting of aluminum, copper, copper-zinc alloy, copper-zincalumirium alloy, nickel, andstainless steel andmixtures thereof. Y

29. The method of treating an abrasive article of manufacture of the type which consists essentially of a coherent mass of granules of an abrasive substance and in which the granules are bonded at their points of contact with a bonding material andV present open spaces between the granules whereby portions of said granules and portions of said bonding materials are exposed, which includes the steps of: immersing the said article of manufacture in a suspension of a metal-leaf pigment in a set- With a 12 table fluid adhesive vehicle, applying pressure thereto so as to force the suspension into the said open spaces, releasing said pressure, removing said article of manufacture, permitting said article of manufacture to drain off excess suspension, and allowing the so treated article to stand until said suspensionreniaining in said article has set.

' chosen from Ithe class consisting ot bide glue, starch glue,

varnish, shellac, alkyd resins, phenolic resins, polyester adhesives, natural rubber adhesives, synthetic rubber adhesives, sodium silicate, and ceramic bodies; said exposed portions of said granules and exposed portions of said bonding material being coated with an adherent layer Y of metal-leaf pigment chosen from the class consisting `of aluminium, aluminum-beryllium all-oy, copper, copperzinc alloy, copper-lead alloy, copper-zinc-lead alloy, beryllium copper, stainless steel, nickel, gold, silver, and platinum and mixtures thereof.

References Cited in the le of this patent UNITED STATES' PATENTS 

8. A COATED ABRASIVE, COMPRISING, A SHEET BACKING, A PLURALITY OF GRANULES OF AN ABRASIVE SUBSTANCE BONDED TO SAID BACKING WITH A BONDING MATERIAL, SAID GRANULES AND SAID BONDING MATERIAL BEING COATED WITH AN ADHERENT LAYER OF A METAL-LEAF PIGMENT. 